Mitsubishi Chariot MX (Diesel) Automatic (1997)

My mitsubishi chariot MX diesel automatic is lacking power, idles rough and black smoke comes out when I rev the engine. My mechanic played about with the fuel pump adjustment (there`s a seal on this adjustment) but still I have the same problem. Sometimes the problem is more evident and sometimes I feel as though the engine is running smooth again. What could be the problem?

Joseph from Malta.